The MOET open herd operated by Genus is an open-nucleus breeding programme, designed to identify superior families and individual females. The herd of 250 cows was established over the period 1987-1990, with embryos from high genetic merit dams in North America. Subsequently, replacement heifers have been bred both from donors selected within the nucleus herd, and from external donors. The herd is milked three times a day, and fed a high-quality complete diet, to maximise production and so facilitate selection. Average NMR 305 day production figures for 440 heifers were 8995 kg, at 3.88% and 3.41% protein. A relatively high proportion of the herd is kept open for embryo production, with 40% of these heifers milking beyond 400 days and 13% for over 500 days. Data from the MOET project are currently being used to model production up to and beyond 305 days, and to establish correlations between performance in early and extended lactations and between lactation length and subsequent performance. These results will be discussed in the context of options for calving interval available to high production-high genetic potential Holstein Friesian herds.